Interleukin-2 (IL2) binds with high affinity to the IL2 receptors overexpressed on activated T lymphocytes in various pathologic conditions. Radiolabeling of IL2 with a positron-emitting isotope could provide a tool for noninvasive PET of activated T cells in immune-mediated diseases. We report the labeling of IL2 with N-succinimidyl 4-¹⁸F-fluorobenzoate (¹⁸F-SFB) for the synthesis of N-(4-¹⁸F-fluorobenzoyl)interleukin-2 (¹⁸F-FB-IL2) and the in vitro and in vivo evaluation of this novel radiopharmaceutical for the detection of IL2 receptor–positive cells by PET. ¹⁸F-SFB was efficiently prepared using a 3-step radiochemical pathway. Purified ¹⁸F-SFB was incubated with IL2 in borate buffer (pH 8.5) and ethanol at 50°C for 10 min. ¹⁸F-FB-IL2 was purified by reversed-phase high-performance liquid chromatography. As in vitro quality controls, a biologic binding assay, sodium dodecyl sulfate polyacrylamide gel electrophoresis, mass spectrometry, and 3-chloroacetic acid precipitation stability tests were performed. Biodistribution studies were performed in BALB/c mice to evaluate the distribution of the tracer in healthy animals. PET experiments were performed in severe combined immunodeficiency disease mice inoculated with phytohemoagglutinin-activated human peripheral blood mononuclear cells (hPBMc). Whole-body images were acquired 30 min after injection of 5–15 MBq of ¹⁸F-FB-IL2. ¹⁸F-SFB was produced with a 34%–38% radiochemical yield. The radiochemical purity after solid-phase extraction purification ranged from 93% to 96%. Conjugation of ¹⁸F-SFB to IL2 yielded ¹⁸F-FB-IL2 as the major product. The radiochemical yield of ¹⁸F-FB-IL2 after high-performance liquid chromatography purification was 25%–35% based on ¹⁸F-SFB. ¹⁸F-FB-IL2 was stable in plasma at 37°C and capable of stimulating T cells to an extent similar to native IL2. A biodistribution study showed highest tracer uptake in the kidneys and bladder due to rapid renal clearance of the tracer. Small-animal PET images showed binding of ¹⁸F-FB-IL2 to activated hPBMc proportional to the number of injected cells. We report the successful labeling of IL2 with ¹⁸F for PET of activated T lymphocytes. ¹⁸F-FB-IL2 is stable, is biologically active, and allows in vivo detection of activated T lymphocytes.